1. Field of the Invention
The present invention relates to an optical add-drop multiplexing apparatus, in particular, to that used in a wavelength division multiplexing communication network.
2. Description of the Related Art
In recent years, as the Internet and cellular phones become common and the functions of terminal units are advanced, the bandwidths of access lines become large and the traffic on the network drastically increases. It seems that such a trend will continue in future. Thus, the infrastructure for communication is being enhanced.
To deal with the increase of the traffic of communications, an optical communications network is being constructed. To accomplish a large capacity of a transmission line, wavelength division multiplexing (WDM) communications system is being introduced. In comparison with the improvement of the technology with respect to capacity of the transmission line, the improvement of the technology with respect to speed of communication nodes (transmission devices) is insufficient. Thus, it is pointed out that the communication nodes will become a bottleneck of the network.
Consequently, in a network using the WDM system for accomplishing a large capacity of a transmission line, communication nodes that allow signals to be processed at high speed are required. The mainstream of the network topology is of ring type. The communication nodes are sometimes accomplished by optical add-drop multiplexing apparatuses.
FIG. 1 is a schematic diagram showing the structure of an example of a conventional optical add-drop multiplexing (ADM) apparatus. Referring to FIG. 1, four input-side inter-station lines and four output-side inter-station lines are connected to the optical ADM apparatus. In this example, the inter-station lines are backbone transmission lines that connect communication nodes and transmit multi-wavelength light (xcex1 to xcexn).
Wavelength demultiplexers 501a to 501d demultiplex multi-wavelength light received through the input-side inter-station lines into signals of individual wavelengths. Cross point switches 502 are disposed corresponding to the individual wavelengths. Input signals are guided to corresponding output ports. FIG. 1 shows a cross point switch 502 corresponding to a wavelength xcex1. Wavelength multiplexers 503a to 503d multiplex outputs of each cross point switch 502 and output the multiplexed signals to the output-side inter-station lines.
The optical ADM apparatus is provided with an add port and a drop port. The add port is used to receive a signal to be added to an inter-station line. The drop port is used to output a signal branched from an inter-station line to an intra-station line. However, a signal that is input through the add port is branched to a set of signals by an optical coupler that functions as an optical splitter and then guided to the cross point switch 502.
Thus, the cross point switch 502 has eight input ports and six output ports and is realized by an 8xc3x976 switch. As shown in FIG. 2, the cross point switch 502 (8xc3x976 switch) is composed of thirty five 2xc3x972 switches and thirteen 1xc3x972 switches.
With the structure, a signal that is input from any input-side inter-station line can be guided to a desired output-side inter-station line or a desired intra-station line. In addition, a signal that is input from an intra-station line through the add port can be guided to a desired output-side inter-station line.
FIG. 3 is a schematic diagram showing the structure of another example of a conventional optical ADM apparatus. In this optical ADM apparatus, an optical switch is divided into a plurality of portions. In other words, the optical ADM apparatus comprises 3xc3x972 switches 511a to 511d, a 4xc3x974 switch 512, 2xc3x972 switches 513a and 513b, and a 4xc3x972 switch 514. The 3xc3x972 switches 511a to 511d are disposed corresponding to the individual inter-station lines. The 4xc3x974 switch 512 are used to switch the route of a signal upon occurrence of a fault. The 2xc3x972 switches 513a and 513b are used to add a signal to a desired inter-station line. The 4xc3x972 switch 514 is used to guide a signal branched from an inter-station line to an intra-station line. With this structure, a signal that is input from any line can be guided to a desired line.
However, when large optical switches are used with the currently available technology, various problems take place. Thus, although the 8xc3x976 cross point switch shown in FIG. 2 can be accomplished, it cannot be obtain the high reliability. In addition, the production cost of the switch becomes very high. Thus, considering these points, at least at the present time, the structure of the optical ADM apparatus shown in FIG. 1 is not preferable.
In the optical ADM apparatus shown in FIG. 3, a 4xc3x974 switch, 3xc3x972 switches, a 4xc3x972 switch, and so forth are used instead of a large cross point switch. However, considering the reliability and the production cost, it cannot be said that they are preferable devices for accomplishing an optical ADM apparatus. In other words, it is desired to accomplish an optical ADM apparatus composed of smaller optical switches than those used so far.
When an optical switch used in an optical ADM apparatus gets defective, the defective optical switch is replaced with a new optical switch. Thus, when a large optical switch is used, the unit to be replaced becomes large. In other words, when a large optical switch is maintained, not only a service with respect to a defective line, but a service without respect thereto may have to be stopped.
An object of the present invention is to provide an optical ADM apparatus with high reliability. Another object of the present invention is to provide an optical ADM apparatus that can be maintained with a minimum of change or stop of services.
An optical add/drop multiplexing apparatus of the present invention is connected to first to fourth input lines, first to fourth output lines, an add line, and a drop line, and this apparatus comprises: four first optical devices for splitting signals that are input from the first to fourth input lines, respectively; four first switches disposed corresponding to the first to fourth output lines, respectively; four second switches disposed corresponding to the first to fourth output lines, respectively; a pair of third switches; a drop unit for guiding a signal split by the first optical devices to the drop line; and an add unit for guiding a signal received from the add line to the second switches. One of the pair of third switches guides a signal received from second switches corresponding to the first and second output lines to first switches corresponding to the third and fourth output lines. The rest of the pair of third switches guides a signal received from second switches corresponding to the third and fourth output lines to first switches corresponding to the first and second output lines. The four first switches output signals split by the first optical devices or signals received from the third switches. The four second switches output signals received from the corresponding first switches or signals received from the add unit to the corresponding output lines.
With this structure, the number of inputs of the first switch is xe2x80x9c2xe2x80x9d, whereas the number of outputs thereof is xe2x80x9c1xe2x80x9d. Thus, the first switch can be accomplished by one switching device. In addition, the number of inputs of each of the second switch and the third switch is xe2x80x9c2xe2x80x9d, whereas the number of outputs thereof is xe2x80x9c2xe2x80x9d. Thus, each of the second switch and the third switch can be accomplished by one switching device. Such switching devices can be produced relatively easily at low cost with high reliability. Thus, an optical Add-Drop multiplexing apparatus with high reliability can be accomplished.